1. Field of the Invention
The present invention relates to aerosol sampling technology, and more particularly to a nozzle plate containing multiple micro-orifices for use in a cascade impactor and a method for manufacturing the same.
2. Description of the Related Art
The Micro-Orifice Uniform Deposit Impactor (MOUDI) invented by MSP Corporation has been widely used for size-classified aerosol sampling. Each stage of the MOUDI consists of a nozzle plate with a plurality of nozzles and an impaction plates to collect particles of a specific size range. By decreasing the nozzle diameter and increasing the air jet speed in the nozzle from the top to the bottom stages, the MOUDI is able to collect particles of subsequently smaller size ranges. In a 10 stage MOUDI, the cutoff aerodynamic diameter of the stage 0 to 10 is 18, 10.0, 5.6, 3.2, 1.8, 1.0, 0.56, 0.32, 0.18, 0.1, 0.056 μm, respectively, and there is a final after filter to collect particles smaller than 0.056 μm. To classify very small particles, the nozzle plates of the last 4 impaction stages, or stage 7 to 10, use 900-2000 micro-orifices with the diameter ranging from 140 to 52 μm to collect particles ranging from 0.32 to 0.056 μm in diameter.
U.S. Pat. No. 6,431,014 disclosed an improved MOUDI design with a series of differential pressure sensors for measuring the pressure drop across the nozzle plates. Additionally, the influence of particle accumulation and blockage in the micro-orifices on the performance of the MOUDI is also briefly discussed. The clogged orifices may cause the cut-point of the impactor to change which leads to measurement errors. The dust accumulation problem in the nozzle can be eliminated by periodic cleaning. However, an improper cleaning method, such as high intensity ultrasonic cleaning, may damage the nozzle plates whose wall thickness to define the nozzle diameter is very thin.
Ji et al. (2006) observed the 6th to 8th stage nozzle plate of a 8-stage MOUDI by using an electron microscope, and the results were published in a journal paper (Ji, J. H., Bae, G. N., Hwang, J., 2006. Observation and evaluation of nozzle clogging in a micro-orifice impactor used for atmospheric aerosol sampling, Particulate Sci. Technol. 24: 85-96). In the study, nozzle clogging caused by particle deposition in the nozzle was observed. The collection efficiency curves were shifted to that corresponding to smaller orifice sizes, and the 50% cutoff sizes were much smaller than those specified by the manufacturer for the three stages with nozzles less than 400 μm in diameter. The pressure drops across the clogged nozzles were also higher than the nominal values given by the manufacturer.
The inventor of the present invention used an optical microscope to observe the micro-orifices of the nozzle plate of the last several stages of the MOUDI. An uneven inner surface of the micro-orifices was observed (see FIG. 11). In the current method, the major part of the nozzle is made by the wet etching process while the final bottom part of the orifice has to be made by laser drilling to define a known orifice diameter. Due to the thickness limitation of laser drilling used to manufacture the orifice, the wall thickness D1 at the bottom side of each micro-orifice is only about 10 μm. This is the main reason why there exists an abrupt step at the bottom of the orifice which renders clogging of particles easily. Besides, this fragile structure prevent the nozzle plates from being cleaned effectively, such as by an ultrasonic cleaner. Improvement of the structure and the shape of the micro-orifices for the nozzle plate is therefore critically needed.